Moneris returns error when trying to void authorization

This is a follow up to https://github.com/Shopify/active_merchant/pull/317

### Problem

We have merchants complaining that voiding authorizations is not working with Moneris.

When we try we get the error:

`Invalid follow_up_amount number`

### TODO

Should we maybe change the implementation of the `void` method to do the $0 capture?